Cal. Public Utilities Code § 739.6

The commission shall establish rates using cost allocation principles that fairly and reasonably assign to different customer classes the costs of providing service to those customer classes, consistent with the policies of affordability and conservation. The cost allocation methodology adopted for gas corporations by the commission in Decisions 86-12-009 and 86-12-010, as supplemented by Decisions 87-05-046 and 87-12-039, is consistent with this policy, and shall be retained by the commission at least until December 31, 1990, except that the commission may modify this cost allocation methodology to address customer hardships and inequities if residential customers as a class are not, on balance, adversely affected and the purpose of the modification is not solely protection of gas corporation revenues. If any gas corporation files a cost allocation application seeking to change that methodology after May 1, 1990, the commission may not issue an order on that application until January 1, 1991.